1. Field of the Invention
The present invention relates to an apparatus for positioning a head of a magnetic storage device, particularly to an apparatus which is able to enhance the precision of the positioning of a head of a magnetic storage device having a magnetoresistance (MR) head.
2. Description of the Related Arts
Magnetic storage devices are widely used as an external storage device of a computer, and, as the capacity of the magnetic storage device increases and the size of the device is reduced, the density of the recording of the magnetic storage device is being progressively enhanced.
In general, in a medium for storage used in a magnetic storage device, the medium is divided circumferentially into a plurality of sectors and radially into a plurality of tracks, and, by specifying a track and positioning the head at a specific area of the medium for storage, it is possible to write data into or read data from the specific area.
Therefore, to promote the enhancement of the density of the recording, it is very important to position the head so as to achieve the correct positioning of the head at the predetermined position on the medium for storage.
In FIG. 1 which shows a functional diagram of the operation of the two phase servo as one of the methods for positioning the head, the medium 100 for storage is rotationally driven by the spindle motor 101, and the writing and reading of the data are carried out by the head 102. The voice coil motor (VCM) 104 is provided for moving the head 102 in the radial direction on the medium 100 for storage.
In FIG. 2, which illustrates the burst pattern recorded in the burst pattern storage region of the medium 100 for storage and used for positioning the head on a predetermined track, the manner of storing four kinds of the burst signals: PosA, PosB, PosC, and PosD along the circumferential direction and along the radial direction is shown.
In FIGS. 3A-3C, which illustrate the waveforms of the outputs of the head which reads the four kinds of the burst patterns at the three positions (a), (b), and (c) in FIG. 2, a signal with the full amplitude is output when the entirety of the head 102 has passed through the test pattern storage region, that is, for example, when the signal PosA is read at the position (a). When a half of the head 102 has passed through the test pattern storage region, that is, for example, when the signal PosA is read at the position (b), a signal with half the amplitude is output. When the head 102 has passed through the region where no burst pattern is stored, that is, for example, when the signal PosA is read at the position (c), no signal is output.
By inputting the signals PosA, PosB, PosC, and PosD through the read and write portion 115 to the demodulation portion 105 and calculating (PosA-PosB) and (PosC-PosD), a demodulation to the two signals PosQ and PosN of the triangular waveform having 90.degree. phase difference with one cycle period constituted by two tracks is carried out.
The signals PosQ and PosN are transmitted to the sensitivity determination portion 106, the position detection portion 107, the speed detection portion 108, and the head position signal generation portion 109.
The sensitivity determination portion 106 determines the gain G which defines the relationship between the amplitude and the track width of the triangular waveform signals PosQ and PosN.
For example, it is possible to determine the gain G based on the following-equation: EQU G=0.25/.vertline.PosQ.vertline.
by positioning the head at the point where the absolute values of PosQ and PosN are equal and using the absolute value of PosQ for the point where the absolute values of PosQ and PosN are equal within the maximum tolerance.
The position detection portion 107 detects the actual absolute position of the head 102 based on the signals PosQ and PosN, and the track number stored in the burst pattern storage region. The speed detection portion 108 detects the speed of the head 102 based on the time differentiation of the absolute position of the head obtained by synthesizing the signal PosQ or PosN and the track number.
The head position signal generation portion 109 divides each of the signals PosQ and PosN into eight sections along the radial direction, extracts the elements in accordance with a predetermined decoding table, and synthesizes the extracted elements so that the head position signal Phd of the saw-tooth waveform is generated.
In FIG. 4, which illustrates the waveforms of the triangular waveform signals PosQ and PosN, and the head position signal Phd, the abscissa represents the track width read by the servo track writer (hereinafter referred to as STW), and the ordinate represents the track width determined based on the output of the reading head of the magnetic storage device.
The actual absolute position of the head 102 detected in the position detection portion 107 is transmitted to the target speed generation portion 110 where the target speed is generated in accordance with the difference from the target track to be accessed. That is, when the difference is relatively large, the target speed is set to be relatively large, while when the difference is relatively small, the target speed is set to be relatively small.
The speed control portion 111, carries out a control calculation, for example, a proportional integral control calculation based on the target speed generated in the target speed generation portion 110 and the actual speed of the head 102 detected in the speed detection portion 108, and outputs the voice coil motor (VCM) driving signal.
The position control portion 112 outputs the VCM driving signal to position the head 102 at the center of the target track, based on the head position signal Phd representing the actual head position generated in the position signal generation portion 109.
The switching portion 113, which switches between the outputs of the speed control portion 111 and the position control portion 112, by which the position of the head is controlled by the speed control portion 111 to quickly bring the head 102 close to the target position when the difference between the target position and the actual position of the head detected by the position detection portion 107 is large, and the position of the head is controlled by the position control portion 112 to position the head 102 correctly at the center of the target track when the head reaches to the target track. The VCM driving signal output from the switching portion 113 is amplified in the power amplifier 116 the output of which is supplied as a current to the VCM 104.
To position the head 102 by the above-described two phase servo device, it is necessary to determine the gain G which is a transformation coefficient which relates the output of the head and the width of the track to make the amplitudes of the triangular shape waves PosQ and PosN to be .+-.0.5 times of the width of the track.
In the detection of the position in the position detection portion 107, since the detection is carried out while the head 102 is moving on the recording medium 100, the error of .+-.1 track with regard to the target track may occur due to an error in reading the track number.
Therefore, the applicant has made the proposal "a method for demodulating position signals and a method for determining position sensitivity" (see Japanese Unexamined Patent Publication (Kokai) No. 8-195044) for enabling to position correctly to the target track and to determine correctly the gain even if the track number is erroneously read, to solve the above-described two problems.
However, to read data from the recording medium 100 in which the data is recorded with high recording density, it is necessary to use a magnetoresistive (MR) head, as the read head, having the read sensitivity of several times greater than that of an inductive head such as a thin film head, and, in the case where the MR head is used, it is necessary to modify the difference in the read sensitivities between the central side (inner side) and the radial side (outer side) of the MR head.
FIG. 5 shows the waveform of the output of the MR head by which the burst pattern stored in the recording medium is read, and FIG. 6 shows the characteristic of the output of the MR head in which the rightwardly rising portion (output increasing portion) and the leftwardly falling portion (output reducing portion) of the waveform are shown, the actually measured values are indicated by the circle marks (.smallcircle.), and the ideal characteristic is indicated by the straight line.
As is seen in FIG. 6, if the position of the head read by the servo track writer (STW) in negative, i.e. if the burst pattern is read by the inner side of the MR head, the actually measured values are outside the ideal characteristic, and hence the reading gain is higher than the ideal characteristic.
Contrary to this, if the position of the head read by the STW is positive, i.e. if the burst pattern is read by the outer side of the MR head, the actually measured values are inside the ideal characteristic, and hence the reading gain is lower than the ideal characteristic.
FIG. 7 shows the reading characteristic showing the reading characteristic of the MR head (which is actually the differential of FIG. 6), in which the high characteristic in the inside and the low characteristic in the outside in the reading characteristic of the MR head are clearly shown.
In the positioning in the magnetic storage device having the MR head with the above-described non-linearity, the MR head is exclusively for the reading, and it is necessary to consider the following points for providing separately a writing head.
That is, when a magnetic storage device is used in the data reading mode, the writing head is required to be positioned at the center of the track, and hence the MR head which reads the burst pattern is required to be positioned offset from the center of the track.
While, when a magnetic storage device is used in the data writing mode, it is necessary only to position the MR head at the center of the track.
Note, the MR head and the writing head can be changed each other.
In relation to this, to compensate for the error caused by the movement of the head along a circular path on the recording medium, there is an apparatus in which the MR head is positioned at the center of the track when the head is used in the writing mode, while the MR head is positioned offset from the center of the track when the head is used in the reading mode.
Thus, in the use of the MR head, since it is necessary to position the MR head not only at the center of the track but also at the position offset from the center of the track, the non-linearity of the MR head has an adverse effect on the positioning of the head.
That is, the following three points become important to position correctly the head in a magnetic storage device having an MR head.
(1) a modification with regard to the non-linearity of the head. PA1 (2) a modification with regard to the shift in the absolute position of the head. PA1 (3) a determination of the gain to establish the correspondence between the output of the head and the width of the track.
A method for compensating the non-linearity of an MR head is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 8-124136, in which the variation rate for compensating the non-linearity of the MR head is calculated based on the head position read by the MR head and the head position read by the STW when the magnetic storage device is produced, the calculated variation rate is stored in a storage medium, and the stored variation rate is read when the magnetic storage device is used, so that the non-linearity of the MR head is compensated.
In this method, therefore, it is not possible to deal with the problem of the change with time of the non-linearity of an MR head.
Also, a method for correcting the shift of the absolute position of an MR head is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 5-174510, in which the method is proposed for ensuring the compatibility of the storage medium by correcting the shift of the absolute position of an MR head, not for enhancing the precision of the positioning of an MR head.
The present invention is proposed in view of the above-described problems, and an object of the present invention resides in providing a head positioning device for a magnetic storage device having an MR head in which the precision of the positioning of the head can be enhanced.